Multidigit all-electronic switching system



y 1950 D. H. RAN SOM 2,506,613

MULTIDIGIT ALL-ELECTRONIC SWITCHING SYSTEM FiledFeb. 7, 1946 5Sheets-Sheet 1 MASTER SHAPER BUSY PULSE DNIDER DIVIDER PHASER no Kc IN VEN TOR. 0/4 V/D H. Eff/V5 0M y 9, 1950 D. H. RANSOM 2,506,613

MULTIDIGIT ALL-ELECTRONiC SWITCHING SYSTEM Filed Feb. '7, 1946 5Sheets-Sheet 2 LOCK-N PHASE coRRezToR zooKcTo 50 KG I CLIPPERmFFEkENTlA- TlNG cIRcuIT AMPLHER IN VEN TOR. 04 W0 H. BAA/60M /l T'TORNEY May 9, 1950 D. H. RANSOM 2,506,613

' MULTIDIGIT ALL-ELECTRONIC SWITCHING SYSTEM l N V EN TOR. 0/] W0 /1.FAA/50M BY v A TTOFNEY y 1950 D. H. RANSOM 2,506,613

MULTIDIGIT ALL-ELECTRONIC SWITCHING SYSTEM Filed Feb. 7, 1946 5Sheets-Sheet 4 OSCILLATOR RINGING ATTORNEY May 9, 1950 D. H. RANSOMMULTIDIGIT ALL-ELECTRONIC SWITCHING SYSTEM Filed Feb. 7, 1946 5Sheets-Sheet 5 AT RNE'Y Patented May 9, 1950 MULTIDIGIT ALL-ELECTRONICSWITCHING SYSTEM David H. Ransom, Montclair, N. J., assignor to FederalTelecommunication Laboratories, Inc., New York, N. Y., a corporation ofDelaware Application February 7, 1946, Serial No. 646,169

18 Claims.

This invention relates to new and useful improvements in communicationsystems and more particularly in electronic switching arrangements forcommunication systems such as telephone exchanges.

The object of the present invention is to provide electronic means forcontrolling switching operations by a plurality of series of pulsesrepresenting, for instance, a plurality of successive digits of a linenumber.

A further object of the invention is to perform by means of anelectronic switch substantially the same kinds of switching operationsas can be performed by means of an electromechanical multi-point switch.

These and other objects of the invention will more clearly appear fromthe following detailed description of certain embodiments thereof andthe appended claims.

In the drawings, Figs. 1-4 diagrammatically illustrate a telephonesystem; Fig. 1 showing the subscribers line and common distributingequipment, Fig. 2 the line finder equipment and the talking circuit,Fig. 3 the line selecting and registering equipment, and Fig. 4 the dialpulse and ringing equipment.

Fig. 5 is a diagrammatic representation of a second type of electronicswitch that may be used in the system, and

Fig. 6 is a plan view of the screen used in said switch.

When a call is initiated at the substation 1 of one of the lines anegative potential is applied from battery 2 over a choke coil 3, awinding of a hybrid coil 4 associated with the line, and the subset tothe top dynode which is assumed to bear designation of a receivingdistributor which, together with a sending distributor 6, is provided incommon for a plurality of groups of subscribers lines which terminate inthe dynodes. In the system here disclosed it is assumed that there aretwenty lines divided into five groups. Any other number of lines may beprovided in any other grouping.

Distributors 5 and 6 are cathode ray tubes provided With the customaryelectron gun structures 8 and 9, the dynode terminals of the lines andanodes, like 1, for collecting the secondary emissions of the dynodes.Only the control grid ll) of the sending tube G'is utilized inthepresent case. The tubes are provided with deflecting means, such asplates H and I2, respectively, which are fed in multiple from a, ZOO-kc.master oscillator 13 connected with the deflecting plates over a 50-kc.frequency divider I l and a IO-kc. frequency divider l5, anda 90 phaseshifter l6.

Whenever the beam of cathode ray'tube'5 engages the "dynode #0 of thecalling line, electrons will flow from the dynode to anode l of thistube and, therefore, a negative pulse 18 will be applied to the grid ofcathode follower and inverter [9. The negative pulse in the cathodeoutput of trio'de i9 is led to the grid of clipper amplifier 20 which isnormally biased to draw current. The amplitude of the negative pulse 2|fed to the grid of this amplifier is so adjusted that it will drive thetube 25 beyond cut-off so that it will clip modulations by thetransmitter at substation l or by pulses produced by dial 22 in thecalling subs'cribers line. The plate output of clipper 20 consists ofpositive pulses 23 fed to the grid of cathode follower 24 and throughthe cathode thereof to conductor 25 which is multipled to the grids ofall the line finder gate tubes like 2%; .provided in the links throughwhich calling and called lines may be interconnected.

The line finder gate 26 'is normally biased far enough beyond cut-off sothat the incomin signal 23 will not affect itsplate output.

The line finder is provided with 'a lock-in oscillator 21 which operatesat a frequency slightly less than that of the master oscillator I3, anddivides this output to a frequency of approximately 50 kc. which ispassed through a clipper and differentiating circuit 28 in the form of asharp positive pulse 29 to a multi-vibrator '30 arranged to synchronizeat approximately 10 kc. The square wave pulses 3! which appear in theoutput of 36 are differentiated in a network 32 and appear as pulses '33in the control grid of clipper gate 34. The constants and bias of gate34 are so adjusted as to produce by the leading edge of pulses 33 in theplate of 34 a, short square negative pulse '35 of approximately fivemicroseconds duration. The trailing edge of pulse 33 is suppressed. Thepulse is passed through, as 35, the cathode of a cathode follower 36,'to the cathode of the line finder gate 26. The amplitude of pulse is soadjusted by a delay gain tube 3.1 that normally the line finder gate 26is not driven beyond cut-off by the positive pulses 23 applied to itscontrol grid. Since the frequency of the line finder lock-in oscillator27 is slightly less than that of the master oscillator [15, the incomingpulses '23 and the local pulses '35 applied to the line finder gate will.drift in time until they occur simultaneously, whereupon av controltube 42 to .drive it beyond cut-off, whereupon a lock-in gate 143 willpass the signal to the lock-in'oscillator'zl, synchronizing the latterwith the master oscillator I3. A phase correotor 44 is provided betweengate 43 and the oscillator 2'! to permit accurate adjustment.

A portion of the output of rectifier 48 is fed to the control grid ofthe delayed gain tube 31 and willgdrive it beyond cut-oil after a fewpulses. The plate resistor 38 of the delayed gain tube is connected withthe screen grid of clipper gate 34. The voltage on this screen will riseand, therefore, increase the amplitude of the pulse 35, which is appliedto the cathode of the line finder gate as a pedestal pulse. The grid ofgate 26 will be driven positive by the incoming pulse 23, and clippingby grid current will occur.

The plate output 39 of the line finder gate is also applied over aconductor 45 to the control grid of an input gate control tube 46. Thetalking circuit has two input gate tubes 41, 48, and two output gatetubes 49, 56 jointly controlled by the input control 46 and the outputcontrol 1 tube The gate tubes are normally biased to cut-ofi on theirsuppressor grids which are connected in pairs to the plate resistors ofthe input and output control tubes; those of gates 41 and 50 to controltube 46 .and those of gates 48 and 49 to output gate control 5|. Thenegative pulse 39 drives the grid of gate control 46 beyond cutoff whichreduces the voltage drop across its plate resistor to zero. This willraise the bias on the suppressor grids of 47 and 50, and permits signalsto pass which may appear on the control grids of these gates.

Returning now to the common circuit affected by the initiation of acall, it is assumed that speech and dial pulses will modulate the linesignal I8 25%. The clipping action of triode 26 will cut offthemodulated portion so that the pulses 23 applied to the line findergates 26 will be uniform. However, a clipper tube 52 to which positivepulses 53 appearing in the plate output of inverter l9 are applied isbiased so that only the modulated portion of the pulse 53 will appear asnegative pulses 54 in its plate circuit. Pulses 54 are applied to thegrid of cathode follower 55 and are transmitted over conductor 56 to thecontrol grids of the input gates 41, 48 of all the links.

Normally a series of negative pulses 54 will be passed by the input gate41 to the dial pulse circuit over conductor 6| before the subscriberstarts dialing. The selective action of the input clipper tube 52 willinterrupt these pulses with each pulse produced by the callingsubscribers dial 22. A low pass filter in the grid of amplifier tube 66to which the output circuit of gate 41 is connected over conductor 6|forms positive pulses 62 appearing therein into low frequency pulseswhich, after amplification, are integrated at 63 and shaped in clippertubes 64 and 65 to form square wave negative pulses 66. The pulses 66are differentiated in a network 61 into pulses 68 which are applied tothe control grid of a unit dialing gate 69 and in multiple therewithover conductor to the control grid of a group dialing gate tube 1|.

The calling subscriber dials two digits; the first representing thegroup or tens designation and the second the units designation of thecalled line.

The bias of the unit and group dialing gates 69, H is so adjusted thatthe leading edge of pulse 68 is suppressed and the trailing edge passedas a negative pulse 12 from the plate of unit dial gate 69over conductor13 to the units register circuit (Fig. 3), and as negative pulse 14appear- 4 ing in the plate of group dial gate H over conductor to thegroup register circuit (Fig. 3). The pulses 12 and 14 are passed onlywhen'the screen grids of the unit and group dial gates are positivelybiased.

Triode 16 of an Eccles-Jordan group digit trigger circuit is normallyconducting and biases the screen grid of unit gate 69 near zero and thescreen grid of grid dial gate H at positive potential. The suppressorgrids of the two dial gate tubes 69 and II are connected in multiple andheld at zero bias by the units digit trigger circuit comprising tubes 18and 19, with the latter normally conducting.

The grid dial gate II will, therefore, pass the first series of dialpulses to set the group register in accordance with the group or tensdesignation of the called line.

When the dialing of the first digit is completed,

the control is transferred to permit the dialing of:

the second digit. This is accomplished by feeding a portion of theoutput of clipper amplifier 65 over a conductor 66 and an integratingnet 8! to the grid of a normally conducting tube 82 which, together witha triode 83, constitutes a digit pulse flip-flop circuit. The firstpulse 66 of a train will transfer conduction from 82 to 83 and thecircuit constants will maintain this condition until the 7 end of aseries of pulses, whereupon 82 will again become conducting. 7

At the end of the first series of dial pulses when 83 is again cut-off,a positive pulse will be sent from its plate over a difierentiatingcircuit 94 to a digit control flip-flop circuit comprising tubes 85 and86, the latter normally conducting. The pulse Will transfer conductionto tube 85 for a period of time determined 'by the constants of thecircuit and a negative pulse 8? will be sent to the group digit controltubes to transfer conduction from tube '16 to H. V

The screen grids of the two dial gate tubes'69 and H being connected tothe plates of i6 and Il, respectively, first, as above described, onlythe group dial gate H and then only the unit dial gate 69. 7

These gates will in turn pass first the pulses 14 to the group registersand then the pulses E2 to the unit register.

After the dialing of the second or units digit, a negative pulse 8'11sent from the digit control 85, 66 will flip the group digit gate sothat 76 will again become conductive and will send out a pulse to theunits digit control #8, 19 to transfer conduction from 79 to E6. Tube 18will now bias v the suppressor grids of both dial gates 69 and H tocut-01f to prevent transients from afiecting the setting of theregisters.

The tube 78 will apply also a bias over conductor 88 to the suppressorgrid of an output gate tube 89 to permit the passage of a ringingsignal, as'will be explained below.

A negative pulse is sent from the plate of 78 over a conductor 98 to aringing control trigger circuit comprisin triodes 9i and 92, with theformer normally conducting. The ringing control circuit contains also aringing gate 93 which is biased to cut-oil until the completionof thedialing when, as above stated, through the agency of the units digittrigger circuit the ringing control tube 9! is made conductive.v Thiswill so bias the control grid of the ringing gate 93 asto make itconductive. Ringing current generated by an oscillator 94 modulates thesuppressor grid of the gate 93 and produces pulses in the plate of thegate 93 which are applied over a conductor 95' to the control grid ofthe output gate 49. Whenever the output gate is functioning, pulses 96will be applied through an amplifier 9'! to the control grid of theoutput gate 09 whose output circuit is connected over conductor 58 withthe control grid ID of the sending distributor 6. This will cause theringing of the called line over distributor t, because the suppressorgrid of 89 is opened when the beam of distributor 5 sweeps the dynode ofthe called line.

The group and units registers are conventional trigger circuitsconnected as binary counters and arranged to receive and store fivedifferent digits representing the group or tens designations of thecalled lines. Obviously, provision may be made for any other grouping.

Each register comprises a pair of tubes such as I00, IQI, I92, I83, I94,I05, I05 and I01, of which the even numbered tubes are normally conducting. The first group dial pulse I4 which arrives over conductor l5 willcause the #1 group register to flip conduction from tube I04 to tubeI05. This in turn will apply a potential over conductor I03 to thecontrol grid of a group selection tube 500 to make it conductive.

The next group dial pulse I4 arriving over con-- ductor it will restoreconduction to tube IE5 which will cut-off the selection tube I09 andpass the pulse to group register #2 over conductor IIEi making I?conductive. A pulse will now be passed from the #2 group register overconductor I I I to operate group selection tube I I2.

A third pulse E4 which arrives over conductor 2'5 will again cause the#1 group register to function and operate the group selection tube I09,the group selection tube H2 having been cut off in the meantime.

The fourth pulse l4 over conductor I restores both the #1 and the #2group registers to normal but will be applied over conductor H3 tooperate the #4 group register II4 (which is like the two other groupregisters). This group register in turn will operate over conductor II5a group selection tube I It.

The fifth group dial pulse I4 arriving over conductor I5 again operatesthe #1 group register and there through the group selection tube I09.

It will be seen therefore that if the first or group digit is 1, thentube I09 is operated, if it is 2, then tube I I2, if 3, then I09 and II2, if 4 then tube H5, and if 5, then tubes H6 and I09.

After the change-over controlled by elements in to 86, the units pulsesI2 arriving over conductor I3 will affect the #1 and #2 units registersin the same manner as was described in connection with the groupregisters. Similarly, the units registers over conductors Ill and I I8will operate units selection tubes II9 and I in the same manner as thegroup selection tubes were operated under the control of the groupregisters. The two units registers and selection tubes make possible theselection of one out of four lines in each of the five groups.

Upon the completion of the dialing, the odd numbered tubes in certain ofthe registers will be conducting. If, for example, the called line #32were dialed, then the #1 and #2 group registers and the #2 unitsregister would be operated, operating in turn the group selection tubesI09 and H2 and the units selection tube I20.

The plates of these selection tubes are connected to common resistancesHI and I22, respectively. When one of the register trigger circuits isflipped to its operating condition, it will bias its associatedselection tube to draw more current, and the amount of current drawn bythe tube will depend upon the resistance in the cathode circuit of thetube and the positive bias applied to its grid. By properly adjustingthe oathode resistors I23, I24, I25, I26 and I2! of the selection tubes,the current flowing through the tubes can be made to vary in intervalsteps. For example, the cathode resistor I23 of tube I09 may beadjusted, so that one milliampere 'of current will flow through the tubewhen positively biased, the resistor I24 so that two 'milliampere's willflow through the tube II2 when positively biased, and I25 so as topermit the flow of four milliamperes through tube II6 when its grid ispositively biased. The current in the output circuit is applied overconductor I2? to control the operation of two electronic switchingdevices I28 and 228, two embodiments of which will now be described withreference to Figs. 5 and 6.

In the electronic switch of Figs. 6 and 7, a cathode I29 is heated by afilament I30 to emit electrons which are controlled by a grid I3I. Theelectrons that pass through grid I3I are formed into a beam by anodesI32, I33 and I34. The beam is constrained to sweep along a circular pathby means of the customary horizontal and vertical deflecting plates I35.The potentials are so adjusted as to direct the beam between thetoroidal electrodes I 36 and I3'I of a condenser. The potentials onthese condenser plates can be adjusted so that the beam will sweep incircles of any desired radius on a screen I38.

As shown in Fig. 6, the screen I38 has angularly and radially displacedcut-out segments I39, I40, MI and I42. The dynodes I43 are aligned withthe cut-outs. Secondary electrons deflected by the dynodes are picked upby a positively charged anode I44 which has extensions (not shown)operating as shields between the individual dynodes I43.

With the four cut-outs I39I4'2 in screen I38, the switch will have atime division of four. However, any other number of cut-outs may beprovided.

The signal may be taken oil the screen I38, the anode I44, or thedynodes I43. The potential applied to the grid I3I may be used tomodulate the beam. The variations in dynode potential will modulate theanode I44 and, conversely, the variations in the potential applied tothe anode I44 may be used to modulate the output of the dynodes I43 whenthe beam sweeps over them.

The switch may be used to afford time division in a multiplex system,the beam being rotated at the recurrence frequency. By adjusting thepotential on the condenser plates I36 and I31, any channel may be chosenand connected with its assigned terminal I43, affording a two-waycommunication since the grid I 3| may be used to modulate the dynode I43and the dynode in turn can be used to modulate the anode I44.

If the number of channels is too great for separation within one tube,then two or more tubes may be connected in tandem. For example, thefirst monoscope would have a circular sweep frequency at the repetitionrate and ten cutouts dividing the time diameter into groups of ten. Theoutput would be taken off the anode I44 and fed to the grid I3! of thenext tube which sweeps at ten times the group frequency and also has tencut-outs in its screen. By applying a potential on the toroidalcondenser I35, I31 in each tube, the desired time channel can beselected.

If it is desired to mark the occurrence of any event in a cycle of.thesweep frequency, thlscan be done in one or more of the tubes.

Two of the switches of the type shown in Fig. 6 are used in. theregistering and selecting circuit of Fig. 3, and will be referred to asgroup monoscope I28 and units monoscope 228. The monoscope I28 bears thesame reference numerals as in Fig. 8, and the monoscope 228 thecorresponding numerals in the two-hundreds.

The beam of the group monoscope I28 is de-. flected by integral steps togive the correct timing pulse period. The beam of the units monoscope228 is rotated. .at 50,000 times per second.

The screen 238, which serves also as an anode,

is divided into segments of one-quarter the total so that fourdefiection'voltages will give four different input signals. Thesesignals are transferred from the dynode 243 of the units monoscope tothe grid I3I of the group monoscope I28 which is rotated at 10,000 timesper second and as five 'time'divisions. During each of these timedivisions, the four units time divisions of mono-. scope 228 can betransmitted. Hence; if proper voltage is applied to the toroidalcondenser plates I36, I31 and 238, 231, any desired group and unitsselection can be performed.

Pulses I45 produced in the dynode I43 of the group monoscope I28 areapplied to the grid of an inverter and cathode follower I46. The anodeoutput of' I 46 is applied over conductor I41 to the suppressor grid ofa pentode I48 controlling the tripping of the ringing current byallowing the passage of a pulse through tube I69 to the control grid ofthe ringing gate 93.

When the called subscriber answers, pulses will be generated in thecircuits associated with the receiving tube 6 in substantially the samemanner as described in connection with the initiation of the call. Thesepulses are then passed through the cathode follower and inverter I9 tothe clipper amplifier 20, cathode follower 24 and then from the cathodeof 24 over conductor I49 to the control grid of the trip ringing tubeI48. The negative pulse appearing in the output circuit of the tripringing tube I48 transfers conduction in the ringing control circuit totube 92 and biases the'ringing gate 93 to cut-off. The application ofthe ringing current from the oscillator 94 is thus stopped and thesubscribers are ready to talk.

The cathode output of the inverter and cathode follower I46 is appliedover conductor I50 to a busy pulse shaper iii and there through to thegrid of a busy gate I52 to cancel positive pulse 23 by a negative pulseI53, thus to prevent a second line finder gate like 26 from operatingwhen the called line answers.

,The pulses on conductor I59 are applied also to the control grid of theoutput gate control tube I. This tube controls the suppressor grids ofthe input gate 4.8 and output gate 49;

A two-way talking circuit is now established between the calling andcalled lines. From the called subscribers line, voice modulated signalsare passed through the receiving distributor 5, the cathode follower andinverter I9, clipper 52, cathode follower 55, conductor 56, to thecontrol grid of the input gate 41. Whenever the suppressor grid of thisinput gate is opened by the control tube 49, the pulse is passed-throughthe gate 41 and a low pass filter I54 to the control grid of output gate49. When the control tube 5I opens the output gate, the pulse is passedthrough amplifier 91, the output gate 89 and conductor 98 to the grid I0of the sending distributor 6, whose beam is at this instant on thedynode in which the called line terminates. In the called line themodulated pulse goes through hybrid coil 4 to operate the receiver inthe subset. I

The path of the modulated speech signals from the calling subscribersline to the called line is the same as above described, except that thesignal is fed through input gate 48 and a low pass filter I55 to thecontrol grid of the output gate 50.

Upon the termination of a call, when the calling subscriber hangs up thevarious circuit elements are released under the control of the delayedgain tube 31. The register circuit and the dial gates which arelocked-in are released under the control of tubes i58, I51 and I58. Thetube I56 is normally conducting. When the calling subscriber starts tocall, the delayed gain tube transmits through its cathode and aconductor 559 a pulse to the grid of release control tube I51 andthrough the plate thereof to the grid of I58 to operate the latter forthe duration of the call. When the calling subscriber hangs up at theend of the conversation, tube I51 again operates and. applies a negativepulse to the grid of the release tube I58 which, by sending a positivepulse over conductor I68 and the conductors E95 and IE2 connectedtherewith, will release all registers and storing circuits to normal.The link is now ready for the transmission of a new call.

WhatIclaim is:

1. In a communication system, a plurality of lines, a plurality oflinks, means for selecting a link upon the initiation of a call on aline, distributor means for cyclically connecting the lines with theselected link, a signal repeater, means controlled over the calling lineand the repeater for selecting a called line, means for transmittinthrough said repeater signals to and from the calling and called linesover the distributor means, means including signals from the callingline for controlling the selecting of the called line, and meansincluding a multi-position electronic switch variably operable inaccordance with the last mentioned selecting operation.

2. The system according to claim 1, and in which the electronic switchcomprises a cathode ray tube having means for producing a. beam ofelectrons, beam deflecting means and a plurality of electrodes on whichthe beam may impinge, and a circuit fo thedeflecting means controlled bythe last mentioned selecting operation.

3. The system according to claim 1, a source of ringin current, andmeans for connecting it with the called line controlled by saidelectronic switch. V

4. The system according to claim 1, a source of ringing current, meansfor connecting it with the called line, and means controlled by theelectronic switch for disconnecting it from the called line.

5. The system according to claim 1, and a circuit for actuating therepeater controlled by said electronic switch. 7

I 6. The system according to claim 1, and means for preventing theselection of a link when the called line answers controlled by theelectronic switch.

7. In a communication system, a pluralit of lines divided into groups, aplurality of links, means for selecting a link'upon the initiation of acall on a line, distributor means for cyclically connecting thelines'with the selected link, a signal'repeaten'means controlled overthe calling line and the repeater for selecting a desired group and anindividual line therein, means for transmitting through said repeatersignals to and from the calling and called lines over the distributormeans, means including signals from the calling line for controlling thedesired selecting operations, and means including a multi-positionelectronic switch variably operable in accordance with the selectingoperations.

8. The system according to claim 7, in which said switch comprises acathode ray tube havin a beam, and means for deflecting the beam inaccordance with a selecting operation.

9. In a communication system, a plurality of lines divided into groups,a plurality of links, means for selecting a, link upon the initiation ofa call on a line, distributor means for cyclically connecting the lineswith the selected link, a sig nal repeater, means controlled over thecallin line and the repeater for selecting a desired group and anindividual line therein, means for transmitting through said repeatersignals to and from the calling and called lines over the distributormeans, means including signals from the calling line for controlling thedesired selecting operations, two cathode ray tubes having beams, meansfor deflecting the beam of one tube in accordance with the groupselecting operation, and means for deflecting the beam of the othe tubein accordance with the individual line selecting operation.

10. In a telephone system, a plurality of lines divided into groups, aplurality of links, electronic distributor means connecting said linescyclically with said links, electronic means for selecting a link uponthe initiation of a call on a line, an electronic signal repeater,electronic register means controlled over the calling line and therepeater for selecting a desired group and an individual line therein,electronic means for transmitting through said repeater selecting andspeech signals to and from the calling and called lines over thedistributor means, means including a. signal controller associated withthe calling line for producing a series of impulses to control i thegroup selecting and a second series of im pulses to control theindividual line selectin op eration of the register, two cathode raytubes each having an electron beam and controlling means for moving itinto any one of a plurality of posltions, a circuit for the controllingmeans of one tube variably operated in accordance with the groupselecting operation of the register, and a circuit for the controllingmeans of the other tube variably operated in accordance with theindividual line selecting operation of the register.

11. The telephone system according to claim 10, and a grid forcontrolling the beam of the first tube controlled by the beam of thesecond tube.

12. The telephone system according to claim 10, and a circuit for therepeater controlled by the beam of the first tube.

13. The system according to claim 10, electronic ring means, and acircuit for the ringing means controlled by the beam of the first tube.

14. The system according to claim 10, electronic ringing means, andmeans jointly controlled .by the register and the first tube forcontrolling the connection of the ringing means with the called line.

15. In a telephone exchange system, a plurality of lines divided intogroups, a plurality of links, means for connecting a calling line with alink, a dial associated with each line for dialing pulses representingthe group to which the called line belongs and also a series of impulsesrepresenting the desired individual called line, group and unitsregistering means associated with the selected link responsive to thetwo series of impulses, a group monoscope responsive to the setting ofthe group register, a units monoscope responsive to the setting of theunits register, and means jointly controlled by the two monoscopes forestablishing a talking circuit between the calling and called lines atpredetermined periods of time.

16. The system according to claim 15, and means jointly controlled bythe two monoscopes for ringing the called line.

17. In a telephone exchange system, a plurality of lines divided intogroups, a dial associated with each line and arranged to produce aseries of impulses representing the group designation and a secondseries of impulses representing the units designation of the desiredcalled line, two electronic registers at the central exchangeresponsive, respectively, to the first and to the second series ofpulses, electronic means for successively associating said registerswith the calling line, an operating circuit for the group register andelectronic means for actuating it in accordance with the group impulse,an operating circuit for the unit register and means for actuating it inaccordance with the units impulses, and electronic means jointlycontrolled by the registers for selecting a called line.

18. In a telephone exchange system, a plurality of lines divided intogroups, a dial associated with each line and arranged to produce aseries of impulses representing the group designation and a secondseries of impulses representing the units designation of the desiredcalled line, two registers at the central exchange responsive,respectively, to the first and to the second series of pulses, acontroller for selectively and successively associating said registerswith the calling line, said controller comprising a group and unit gatepentode and a group and unit digit control trigger circuit, an inputcircuit for applying pulses produced by the dial of the calling line tothe control grids of the group and unit gate pentodes in multiple, anoperating circuit for the group register connected with the anode of thegroup .pentode, an operating circuit for the unit register connectedwith the unit pentode, means in the group trigger circuit for normallybiasing the screen grid of the unit dial gate to out-ofi and a screengrid of the group dial gate so that signals may pass therethrough, amultiple circuit for the suppressor grids of both pentodes controlled bythe units trigger circuit, means for connecting said pulse input circuitwith the group trigger circuit upon the termination of each series ofdial pulses, and means for operating the units trigger circuitcontrolled by the group trigger circuit.

DAVID H. RANSOM.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,263,369 Skillman Nov. 18, 19412,266,671 Wolf Dec. 16, 1941 2,379,715 Hubbard July 3, 1945 2,387,018Hartley Oct. 16, 1945

